Clickers
aka classroom response systems (CRS), aka audience response systems (ARS)
These are little remote control-like devices that each student purchases (or is provided). The teacher asks a question, and the students enter their answer, and the teacher can display a graph of the answers. This is a type of formative assessment. There is a good bit of research now showing that clickers (as well as other kinds of formative assessment) are very effective if used correctly.
There are really tons of books and journal articles about clickers now, if you search for clickers or one of the synonyms above at google books or google scholar.
Alternatives to Clickers
Clickers are quite expensive and have a limited focus. You can mainly just do multiple choice questions with them, for example. Make sure you let students know that bringing the clickers to every class is a requirement, too.
If the students have laptops or cell phones, there are alternatives that would work. I don't know specific ones off the top of my head (other than using twitter/edmodo or a twitter poll or something like surveymonkey or a quiz in your LMS), but feel free to add any to this wiki page.
Response cards - Alternatively, you can have students hold up colored pieces of paper to indicate their response. Red for A, Blue for B, Green for C, etc. They raise their hands holding the colored paper they choose, and you can quickly see what the answer distribution is. There is a lot of research backing their effectiveness as well:
Clickers
aka classroom response systems (CRS), aka audience response systems (ARS)
These are little remote control-like devices that each student purchases (or is provided). The teacher asks a question, and the students enter their answer, and the teacher can display a graph of the answers. This is a type of formative assessment. There is a good bit of research now showing that clickers (as well as other kinds of formative assessment) are very effective if used correctly.